As disclosed in Japanese Laid-Open Patent Publication No. 61-180666, there has been employed in the art a molten metal supplying apparatus for supplying molten metal from a molten metal holding furnace into a mold or molten metal receiving member through a distribution tube system which has an electromagnetic pump. In one arrangement, the electromagnetic pump is disposed below the surface level of molten metal in the holding furnace According to another system, the electromagnetic pump is positioned above the surface level of molten metal in the holding furnace, and air pressure and the electromagnetic pump are relied upon to supply the molten metal from the holding furnace into the mold.
In the former structure, the distribution tube connecting the holding furnace and the molten metal receiving member is required to be adjusted to the construction of the molten metal receiving member, i.e., to the position of the mold or the like, and hence the distribution tube is complex in structure and many joints are employed. Accordingly, the danger of leakage of the molten metal somewhere in the distribution tube exists. Since the distribution tube is located below the surface level of the molten metal in the holding furnace, if the distribution tube has to be inspected or serviced, all of the molten metal must be removed from the holding furnace. Such a process is time-consuming, and hence is not preferable from the standpoint of the production efficiency.
The latter construction is not versatile as it can only be used with low-pressure die-casting machines. Inasmuch a the molten metal receiving member is positioned higher than the surface level of the molten metal in the holding furnace, when the molten metal is supplied from the holding furnace to the molten metal receiving member, part of the molten metal is brought into contact with air and hence oxidized. After the molten metal has been supplied to the mold, an excessive amount of supplied molten metal cannot fully be returned into the holding furnace, and is partly deposited on the inner peripheral surface of the distribution tube. The deposited molten metal is oxidized by contact with air, and the oxidized deposit narrows the molten metal flow passage in the distribution tube. As a result, the rate of flow of the molten metal through the distribution tube in a next casting process is varied. With the conventional molten metal supplying apparatus of the type described above, the molten metal holding furnace and the molten metal receiving member are fixed in position. When the molten metal is supplied, therefore, the distribution tube is expanded and contracted because of an increase in the temperature of the distribution tube. As a consequence, the durability of packings or sealing members attached to the joints of the distribution tube is reduced.